Ecological communities can affect transmission pathways of parasites and
pathogens, ultimately affecting disease dynamics. While the community
composition of less competent decoy hosts is known to affect diseases in
focal hosts, it remains poorly understood whether such diversity effects
also exist when non-host organisms remove free-living parasite stages,
e.g. by predation. In response surface design laboratory experiments, we
investigated non-host diversity effects on the removal of cercarial
stages of trematodes, ubiquitous parasites in aquatic ecosystems. In all
three combinations of two non-hosts at four density levels, the addition
of a second non-host did not generally result in increased parasite
removal but neutralised, amplified or reduced the parasite removal
exerted by the first non-host, depending on the density. These complex
non-host diversity effects were probably driven by intra- and
interspecific interactions and suggest the need to integrate non-host
diversity effects in understanding the links between community diversity
and disease risk.